The present invention relates to photometer means as adapted to a photographic system for a video display including compensation for reciprocity failure characteristics of the film as well as film speed.
Exposure controls which are correlated to various parameters of photographic film and the amount of light flux incident thereon are well known. Such systems are often exposure controls of the capacitance type in which a capacitor is charged at a rate proportional to the illumination to which the film is subject, for example, by exposing a photocell to the same illumination. The charge on the capacitor is the time integral of the current in the capacitor and analogously the film exposure is the time integral of light flux incident on the film. Therefore, the capacitor integrates current with respect to time as the photographic film integrates incident light flux with respect to time. Thus, at a given predetermined level of voltage on a capacitor, representative of the predetermined desired integral of light flux, the camera shutter may be closed.
In such an arrangement, the speed of the film (ASA) is an analog of the capacitance, that is a capacitance having a given charge rate is used in conjunction with a given ASA film having a given rate at which it becomes light saturated. Additionally, some prior exposure controls take into account reciprocity failure characteristic of film. Reciprocity failure is a property of photographic emulsion by which its light sensitivity decays during time of exposure. That is to say the curve of sensitivity versus exposure time is nonlinear. For long exposure times as are typical, for example, in photomicrography of the type presently illustrated, this change in characteristic becomes significant.
In those instances where photomicrography is done on a scanning electron microscope wherein the photograph taken is of video display, procedures taken to photograph vary somewhat from those followed in other photomicrography.
In the instance of scanning electron microscopy, one normally predetermines the exposure time which he desires to use in generating a photomicrograph. This determination of exposure time is usually dependent upon such things as the characteristics of specimen charging, specimen motion, or other characteristics which are peculiar to photomicrographs prepared in scanning electron microscopy.
It may be thus noted that rather than exposure time being calculated as in the conventional sense based upon light flux incident on the film, the reverse procedure is actually followed. One must then follow procedures to match the light flux incident upon the film to the chosen exposure time. In conjunction with such matchings, one must give significant consideration to the characteristic of reciprocity failure in that photo exposure times in certain SEM photomicrography are substantial. These times may amount to 60 or more seconds per photograph.
Reciprocity failure is of concern for photomicrographs which are prepared from multiple rasters of low brightness which are integrated by the photographic film over a relatively long exposure time. See, for example, the multiple interlace system referred to in U.S. Pat. No. Re. 28,153 assigned to the Assignee of the present application. Single scan raster photography (such as is used by most of the conventional SEM's) does not suffer the problem even though the total exposure time may be the same. In these single scan systems, the illuminating spot traversing the film passes each spot (transits the raster) only once so the film exposure, considered on a spot-by-spot basis, occurs only once over a very short period of time and thus not generating the reciprocity problem.
With the exception of reciprocity failure and gamma (which is not involved in the calculations of the present invention) photography can be considered a linear process. However, by custom, and for practical considerations (ease of computation of parameters) most of the factors involved in photography are presented in multiples of two (e.g. in binary function). For example, typical camera shutter speeds are approximately multiples of two (e.g. notations of: 1, 2, 5, 10, 25, 50, 100, 250, 500 and 1,000 indication fractions of seconds). Likewise, camera F-stops although not always so marked, are factors of two in amount of flux reaching the film (e.g. notations of: 2; 2.8; 4; 5.6; 8; 11; 16; 22; and 32.).
With the present improvements while making the control functions for the scientific instrument more convenient and usable to the operator, one observes a collateral benefit of being able to use less expensive linear devices in such as meters as opposed to more highly sensitive or sophisticated logarithmic instruments.